Polarizing plates used for various image display devices such as liquid crystal display devices, plasma display devices and electroluminescence devices have a structure including a polarizer formed with a polyvinyl alcohol-based film, and a protective film formed on at least one surface of the polarizer. Meanwhile, polyvinyl alcohol-based adhesives are mainly used as an adhesive for a polarizing plate for attaching the polarizer and the protective film.
However, polyvinyl alcohol-based resin adhesives are a water-soluble polymer, therefore, have a problem in that the polarizer and the protective film are peeled off since the adhesive is dissolved under high humidity conditions.
In view of the above, methods of modifying a polyvinyl alcohol-based resin by adding an acetoacetyl group thereto, copolymerizing a hydrophobic vinyl monomer such as ethyl acrylate, methyl acrylate, acrylic acid and methacrylic acid, or mixing a cross-linking agent such as glyoxylate or an amine metal salt to a polyvinyl alcohol-based resin have been proposed.
Polyvinyl alcohol-based adhesives prepared using the methods described above tend to have somewhat improved water resistance compared to when polyvinyl alcohol is used alone, however, there is still a problem in that water resistance and adhesive strength are not sufficient when the adhesives are used in high-speed production lines. In addition, modified polyvinyl alcohol-based resin adhesives have a problem in that stains are resulted under alkaline pH conditions, and methods of mixing a cross-linking agent also have a problem in that optical properties of a polarizing plate are degraded depending on the amount of the cross-linking agent used.
Accordingly, adhesives for adhering a polarizer and a protective film having an excellent adhesive property and water resistance while maintaining optical properties of a polarizing plate have been required.